Originally Posted by
keisari
I thought that they are determined by the airline with the longest segment based on star alliance rules
Flights to/from the US are the exception. The US follows the rules of the first (check-in) carrier. The rest of the world, or most of it, follows the most-significant carrier rule: basically the carrier with the longest segment determines the rules.
I am not sure about this... my flights are almost all domestic US. Almost all exceptions are to/from the US.